Aerosol inhalation equipment is often used in medical facilities for generating aerosol mists for diagnostic and therapeutic procedures. Such devices are especially useful in pulmonary therapy for pneumonia and for introducing radioactive vapors for diagnosing diseases.
The engine of most aerosol-generating equipment is the nebulizer, a device which mixes pressurized air or oxygen with diagnostic or therapeutic fluids to create an aerosol mist. During operation, the liquid to be aerosolized is placed in a liquid reservoir in the nebulizer. Air under pressure enters the system and acts to draw the liquid up a delivery tube to an aerosol exit orifice, similar to a jet pump. At the aerosol exit orifice, the fluid is atomized into a fine mist. Larger drops that are produced in the mist impinge on baffles above the aerosol exit orifice where they drain back into the reservoir of the nebulizer. Smaller drops are entrained by the air and are carried through the delivery system to the patient's lungs. A typical nebulizer is disclosed in Bordoni et al., U.S. Pat. No. 4,823,784.
The overall effectiveness of a nebulizer depends largely upon the distribution and size of the droplets produced. Droplets larger than 3.5 micrometers generally do not leave the nebulizer and run back into the bowl to be atomized again. Droplets between about 1.5 and 3.5 micrometers often collect on the walls of the delivery system and frequently settle onto the lips, mouth, or bronchial tubes of the patient without ever reaching the alveolar, often referred to as the "deep lung".
A 1987 study performed on one aerosol inhalation system indicated that only 25% of the liquid initially charged into the nebulizer actually reached the patient's lungs during a seven minute exposure. Typically, only 66% of this amount actually remains in the deep lung of the patient; the other 33% is exhaled. Therefore, only about 16% of the liquid therapeutic or diagnostic substance charged to the nebulizer is ever used by the patient. The remainder is wasted, or winds up contaminating the environment. Consequently, much of the liquid charge loaded into the nebulizer is provided only to ensure that a proper dose can be received in a reasonable exposure time.
Since most aerosol inhalation devices are currently made for one-time patient use, and are thereafter disposed, any medication or diagnostic fluid remaining in the device becomes waste. Nevertheless, decreasing the amount of fluid required for delivering a prescribed amount to a patient's lung while increasing the delivery efficiency of the fluid has been hard to achieve. With respect to radioactive therapeutics and diagnostics, there is also a great need to decrease the treatment times so as to minimize radiation doses to both the patient and nearby healthcare workers.
Despite these recognized deficiencies of current nebulizer devices, there has been very little improvement in the actual performance of modern systems. Most of the more recent variations have been in the nebulizer design, such as the placement of baffles, variations in orifice size and other structural elements which are somewhat unrelated to increased delivery. There has been very little research conducted to improve the particle size distribution and control through nebulization techniques rather than through baffling. There has also been few changes made in the recovery of medication and trapped moisture to reduce waste, and to improve production efficiency.
Even though modern investigators have presently correlated the mass-median-drop size (MMD) produced by a nebulizer to the properties of the liquid and the delivery system, there is currently no way to account for the effect of structural variations on the drop size produced. Moreover, previous studies have also shown that various commercial nebulizers produce essentially the same aerosol, but the structural differences between the nebulizers, such as the addition and placement of a baffle, caused selective losses of larger particles. This method of drop size reduction, by itself, is generally unsatisfactory and inefficient because the drop size is reduced at the expense of reducing the rate of drug delivery.